Sensorless motor control

ABSTRACT

An electric motor control system includes a power inverter and control circuitry configured to control the power inverter either according to a target voltage in a voltage-based control mode or according to a target current in a current-based control mode. A controller is operable to switch operation of the control circuitry between the voltage-based control mode and the current-based control mode. The controller may be configured to operate the control circuitry in the current-based control mode at lower motor operating speeds where stator current margin is of greater significance, and to operate the control circuitry in the voltage-based control mode at higher motor operating speeds where stator voltage margin is of greater significance.

FIELD

Embodiments of the present invention relate to systems and methods for controlling electric motors.

BACKGROUND

Electric motors commonly include a stationary component called a stator and a rotating component called a rotor. The rotor rotates within (or around) the stator when the motor is energized with a driving waveform. Induction motors, sometimes referred to as asynchronous motors, are a type of electric motor wherein power is supplied to the rotor by means of electromagnetic induction rather than by means of direct electrical connections to the rotor.

As with synchronous motors, the driving waveform supplied to an induction motor's stator creates a magnetic field that rotates in time with the AC oscillations of the driving waveform. The induction motor's rotor rotates at a slower speed than the stator field. This difference in rotational speed, also referred to as “slip,” “slip frequency,” or “slip speed,” results in a changing magnetic flux in the rotor windings that induces currents in the rotor windings. The induced current in turn generates magnetic fields in the rotor windings that oppose the rotating magnetic field created by the stator, thereby inducing rotational movement in the rotor. The rotor accelerates until the magnitude of induced rotor current and rotor torque balances the applied load. Since rotation at synchronous speed would result in no induced rotor current, an induction motor always operates at less than synchronous speed during normal forward operation.

When the frequency of the driving waveform falls below the rotor frequency, such as where the programmed speed of the motor is reduced by a motor controller during operation, the rotating magnetic field created by the stator induces rotational pressure on the rotor that opposes the rotor's movement and reduces motor speed. In this braking mode of operation, the inertia of the rotor and applied load induces voltage in the stator that may energize external motor components, such as a DC bus supplying power to the motor.

A known method of controlling electric motors is field oriented control or “FOC.” Field oriented control involves controlling a motor using three motor input variables including voltage magnitude, voltage angle and frequency. Because stator current is closely related to output torque and other operating characteristics of the motor, it is desirable to use field oriented control methods to operate the motor according to a target stator current. At high motor speeds, however, manipulating the stator current using field oriented control becomes less effective because the motor operates at or near a maximum voltage level, eliminating or reducing one of the input variables (voltage magnitude).

The above section provides background information related to the present disclosure which is not necessarily prior art.

SUMMARY

Embodiments of the present invention solve the above described problems by providing improved systems for controlling electric motors that are capable of operating in different operating modes according to the operating speed of the motor.

A motor control system constructed in accordance with an embodiment of the present invention comprises a power inverter, control circuitry configured to control the power inverter either according to a target voltage in a voltage-based control mode or according to a target current in a current-based control mode, and a controller for switching operation of the control circuitry between the voltage-based control mode and the current-based control mode.

In a related embodiment, the control circuitry includes a plurality of regulators for generating and/or adjusting motor control signals. A first regulator receives a differential speed signal and generates a torque producing component i_(qref) of a reference current signal based on the differential speed signal, the differential speed signal representing a difference between a reference motor speed and an estimated motor speed. A second regulator receives a reference speed signal and generates a flux producing component i_(dref) of the reference current signal.

A third regulator receives a differential i_(q) signal and generates a torque producing component V_(q) of a reference voltage signal, the differential i_(q) signal representing a difference between i_(qref) and an estimated actual torque producing component i_(q) of a motor driving waveform. A fourth regulator receives a differential i_(d) signal and generates a flux producing component V_(d) of the reference voltage signal based on the differential i_(d) signal, the differential i_(d) signal representing a difference between i_(dref) and an estimated actual flux producing component i_(d) of the motor driving waveform.

In another embodiment of the invention, a method of operating a motor control system comprises generating a first motor drive signal according to a target current if a reference motor speed is within a first speed range and generating a second motor drive signal according to a target voltage if the reference motor speed is within a second speed range. The method further comprises automatically switching operation of the motor control system between generating the first motor drive signal and generating the second motor drive signal as the target speed changes between the first speed range and the second speed range.

This summary is provided to introduce a selection of concepts in a simplified form that are further described in the detailed description below. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Other aspects and advantages of the present invention will be apparent from the following detailed description of the embodiments and the accompanying drawing figures.

DRAWINGS

Embodiments of the present invention are described in detail below with reference to the attached drawing figures, wherein:

FIG. 1 is a block diagram of a motor system constructed in accordance with embodiments of the invention.

FIG. 2 is a schematic representation of control circuitry for use with the motor system of FIG. 1.

FIG. 3 is a schematic representation of an alternative embodiment of the control circuitry of FIG. 2.

FIG. 4 is a graphical representation of certain operating and performance characteristics of the motor of FIG. 1.

The drawing figures do not limit the present invention to the specific embodiments disclosed and described herein. The drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the invention.

DETAILED DESCRIPTION

The following detailed description of embodiments of the invention references the accompanying drawings. The embodiments are intended to describe aspects of the invention in sufficient detail to enable those skilled in the art to practice the invention. Other embodiments can be utilized and changes can be made without departing from the scope of the claims. The following detailed description is, therefore, not to be taken in a limiting sense. The scope of the present invention is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled.

In this description, references to “one embodiment”, “an embodiment”, or “embodiments” mean that the feature or features being referred to are included in at least one embodiment of the technology. Separate references to “one embodiment”, “an embodiment”, or “embodiments” in this description do not necessarily refer to the same embodiment and are also not mutually exclusive unless so stated and/or except as will be readily apparent to those skilled in the art from the description. For example, a feature, structure, act, etcetera described in one embodiment may also be included in other embodiments, but is not necessarily included. Thus, the present technology can include a variety of combinations and/or integrations of the embodiments described herein.

Turning now to the drawing figures, and initially FIG. 1, an electric motor system 10 constructed in accordance with embodiments of the invention is illustrated. The motor system 10 broadly includes an electric motor 12, a power source 14, and a motor control system 16 for receiving power from the power source 14 and generating a driving waveform to power the motor 12 according to a reference motor speed (ω_(ref)) communicated to the control system via a reference speed input 18. The reference motor speed corresponds to a desired or target operating motor output speed and may be generated by an external motor control mechanism (not illustrated) configured to operate the motor 12 according to a particular implementation of the invention. The motor control system 16 includes control circuitry 20 configured to receive input power from the power source 14, receive a reference motor speed signal from the reference speed input 18, create the driving waveform, and communicate the driving waveform to the motor 12. A controller 22 manages certain aspects of the operation of the control circuitry 20, including switching between a voltage-based control mode and a current-based control mode, as explained below.

The power source 14 may be a conventional alternating-current power source, such as a standard 115V or 230V source available in residential and commercial buildings via standard electrical outlets. The motor system 10 may be part of an appliance drive system, such as a washing machine or a clothes dryer, that includes additional electrical and mechanical components not illustrated or described herein.

The motor 12 may be an electric induction motor and, in one exemplary embodiment, is a three-phase, four-pole AC induction motor rated to operate at a maximum voltage of between 190 and 200 volts and a maximum current of between 4 and 6 amps. The motor 12 may be configured to operate within a normal operating range of, for example, between 0 and 3,000 rpm, and may be capable of operating at speeds well beyond the normal operation range, such as up to 17,000 or 18,000 rpm. Motor operation beyond the normal operating range may be characterized by progressively diminishing performance characteristics, such as decreased torque, as explained below.

Various operating and performance characteristics of the motor 12, including stator voltage (V_(s)), maximum operating current and output torque, are illustrated in the graph of FIG. 4 across the operating speed range of the motor 12. As shown in FIG. 4, motor performance characteristics differ according to whether the motor is operating in a normal operating region or a field weakening region. The normal operating region corresponds to a motor output speed of zero up to a nominal operating speed 24, which may be, for example, 3,000 or 4,000 rpm. The field weakening operating region corresponds to the nominal speed 24 up to a maximum motor speed 26, which may be, for example, about 17,000 or 18,000 rpm. When operating in the normal operating region, the speed of the motor 12 generally follows a linear pattern and is proportional to the stator voltage (V_(s)), while torque is generally constant. In the field weakening region, the stator voltage remains at or near a maximum operating voltage and the torque diminishes as the motor output speed increases. The maximum operating current is defined by limitations of the motor 12, the control circuitry 20, or both. While the maximum operating current generally remains constant across all motor operating speeds, the motor stator current (i_(s)) will typically fluctuate with changing operational parameters, such as changes in motor speed and load torque. The total stator current (i_(s)) includes a torque producing component (i_(sq)) and a flux producing component (i_(sd)), whose relationship is defined by the following equation: i _(s)=√{square root over ((i _(sd))²+(i _(sq))²)}{square root over ((i _(sd))²+(i _(sq))²)}

For purposes of the present teachings, the motor 12 generally operates in three different output speed ranges including low speed, medium speed and high speed. The low speed range generally corresponds to the normal operating region, the medium speed range generally corresponds to a lower portion of the field weakening region, and the high speed range general corresponds to an upper portion of the field weakening region, as indicated in the graph of FIG. 4. The motor speed corresponding to the boundary between the medium and high speed ranges may be defined as a particular motor speed, such as 8,000 rpm or 10,000 rpm, or as a percentage of a maximum motor speed, such as 50%, 60% or 70% of the maximum speed 26. Alternatively, the boundary may be defined in terms of the stator current, such as a particular value of i_(sd) or i_(sq) or a difference between i_(sd) and i_(sq). Further yet, the boundary between the medium and high speed ranges may be defined in terms of the torque, such as a particular torque value or percentage of maximum torque. The boundary between the medium and high speed ranges is relevant in that may be used by the controller as a threshold speed for switching between the voltage-based control mode and the current-based control mode, as explained below. In some embodiments, the boundary between the medium and high speed ranges is defined as a rotor speed at which the stator voltage amplitude is at or near the inverter voltage margin. When the motor is operating at or near the inverter voltage margin, fluctuations in the inverter voltage margin due to, for example, variations in the DC bus voltage make current-based motor control impractical.

The controller 22 includes digital logic configured to interact with and control certain components of the control circuitry 20. The digital logic may include discrete logic components, programmable logic devices and/or general purpose computer processors such as microcontrollers or microprocessors. In some embodiments of the invention, the controller 22 is or includes a computer processor configured to execute a computer program for directing the control circuitry 20 to operate in either the current-based control mode in the low and medium speed ranges or in the voltage-based control mode in the high speed range.

Thus, aspects of the invention may be implemented with one or more computer programs stored in or on computer-readable media residing in or accessible by the controller 22. The computer program comprises a series of executable instructions for implementing logical functions in the controller 22. The computer program can be embodied in any non-transitory computer-readable medium for use by or in connection with the controller 22 wherein the controller fetches instructions from the computer-readable medium and executes the instructions. As used herein, a “computer-readable medium” can be any non-transitory means that can store the program for use by or in connection with the controller. The computer-readable medium can be, for example, but is not limited to, an electronic, magnetic, optical, electro-magnetic, infrared, or semi-conductor system, apparatus, or device. More specific, although not inclusive, examples of the computer-readable medium would include the following: an electrical connection having one or more wires, a random access memory (RAM), a read-only memory (ROM), and an erasable, programmable, read-only memory (EPROM or Flash memory).

Turning now to FIG. 2, certain components of the control circuitry 20 are illustrated in greater detail. A rectifier 30 converts AC power received from the power source 14 to DC power that energizes a DC bus 32. The rectifier 30 may include virtually any type of rectifier circuit, but in various preferred embodiments is a simple non-regenerative rectifier, such as a diode bridge rectifier. A signal shunt 34 and voltage sensor 35 are coupled with the DC bus 32 and used to detect voltage and/or current levels on the DC bus 32 for use in feedback loops, as explained below in greater detail. The signal shunt 34 may be, for example, a resister, while the voltage sensor 35 may be a voltage divider circuit. One or more capacitors 36 are also coupled to the DC bus 32 to minimize voltage ripple on the bus 32 and to store regenerative energy in the bus 32 during braking.

Inverter circuitry 38 includes pulse width modulation (“PWM”) control logic 40 and a three-phase inverter 42 that converts DC power from the DC bus 32 to driving waveforms that are communicated to the motor 12. The PWM control logic 40 receives two input signals (V_(αref) and V_(βref)) representing two-phase stationary reference frame voltage values V_(α) and V_(β), and converts the input signals to inverter control signals that drive the inverter to convert power from the DC bus 32 to the driving waveforms applied to the motor stator windings. The voltage magnitude, voltage angle and frequency of the driving electric waveforms are manipulated according to the reference motor speed ω_(ref) to control operation of the motor 12. When the motor 12 is operating at a constant speed or accelerating, it has a positive slip frequency and generally consumes energy. When the motor 12 is decelerating (braking), it has a negative slip frequency and generates energy that is dissipated in the motor 12 (as rotor and stator losses) and that energizes the inverter 42 and the DC bus 32, thereby increasing the voltage in those components beyond the voltage supplied by the rectifier 30. The increased voltage on the DC bus 32 may contribute to braking by further increasing stator voltages and losses in the motor 12, as explained below in greater detail.

When the switch 28 is in a first position (represented by the broken lines in FIG. 2), the control circuitry 20 operates in the current-based control mode according to principles of indirect field oriented control (IFOC). In the current-based control mode, the control circuitry 20 drives operation of the power inverter 42 according to a target stator current. The IFOC motor control method involves controlling AC motor operation by means of three motor input variables: voltage magnitude, voltage angle and signal frequency. These variables are manipulated according to the reference speed ω_(ref) via two control/feedback loops that independently control the speed (frequency) and strength of the rotating magnetic field induced in the motor's stator.

The reference motor speed (ω_(ref)) signal is received at the left side of the circuit diagram in FIG. 1. The ω_(ref) signal may be received from an external motor controller, such as an appliance motor controller configured to control the speed and operation of the motor according to operational parameters of the particular appliance. An estimated rotor speed, ω_(r), is subtracted from the reference speed and the difference is communicated to a first proportional-integral controller 44 configured to generate a torque-producing component i_(ref) of a target stator current. The estimated motor speed (ω_(r)) is also communicated to a field weakening controller 46 configured to generate a flux-producing component i_(dref) of the stator current. The field weakening controller 46 decreases the value of i_(dref) when the reference motor speed is in the field weakening region of motor operation.

The i_(qref) signal is communicated from the first proportional-integral controller 44 to a DC voltage limiter circuit 48 that is configured to limit the maximum negative value of i_(qref) based on the voltage limitations of the motor 12, inverter 42 and/or capacitor 36. The value of i_(qref) is negative during motor braking when the reference motor speed is less than the actual rotor speed. When braking, regenerative energy from the motor 12 increases the voltage in the inverter 42 and the DC bus capacitor 36, as explained above. The voltage limiter circuit 48 prevents the negative value of i_(qref) from exceeding a level corresponding to the maximum voltage of the motor, inverter and/or capacitor. The voltage limiter circuit 48 receives a first signal V_(dcmax) representing a maximum voltage level, a second signal V_(dc) representing an estimated or measured voltage on the DC bus 32, and subtracts the value of V_(dc) from V_(dcmax). An integrator controller 50 integrates the difference and communicates the result, −i_(qrefmax), to a first current limiter component 52 that limits i_(qref) to the value of −i_(qrefmax).

A current limiter component 54 receives both i_(qref) and i_(dref) signals and imposes a limit on the signals corresponding to a maximum stator current as defined by a maximum rating of the motor 12 and/or the inverter 42. Thus, while the voltage limiter circuit 48 imposes a maximum negative value of i_(qref) during braking to regulate regenerative voltage, the current limiter component 54 imposes a maximum positive value of i_(qref) and i_(dref) during normal operation. Measured values of the torque-producing component i_(q) and the flux-producing component i_(d) of the actual stator current are subtracted from i_(qref) and i_(dref), and the resulting difference is communicated to second 56 and third 58 proportional-integral controllers which calculate rotating two-phase voltage signals V_(qref) and V_(dref).

An inverse Park conversion circuit 60 converts the V_(qref) and V_(dref) signals to stationary two-phase signals V_(αref) and V_(βref) which are received by the PWM control logic 40 of the inverter circuitry 38. The PWM control logic 40 generates a plurality of inverter control signals based on the stationary two-phase signals V_(αref) and V_(βref), as explained above, and communicates the control signals to the inverter 42.

In the current-based control mode, the control circuitry 20 uses feedback loops to generate the motor control signals. The shunt 34 and sensor 35 detect current and/or voltage on the DC bus 32 and communicate the information to a converter component 62 that generates a plurality of signals representing measured values of the instantaneous stator current and/or voltage. A Clarke conversion circuit 64 converts the measured stator current and/or voltage values to two-phase, stationary values i_(α) and i_(β), which are then converted to two-phase rotating signals i_(q) and i_(d) by a Park conversion circuit 66. The i_(q) and i_(d) signals are combined with the i_(qref) and i_(dref) signals, as explained above. The Clarke conversion circuit 64 may also generate two-phase, stationary values V_(α) and V_(β) that are used by a speed estimator 68 to estimate the rotor speed ω_(r).

The speed estimator 68 calculates an estimated rotor speed, ω_(r), based on characteristics of the power communicated to the stator, such as the estimated values i_(α), i_(β), V_(α) and V_(β) generated by the Clarke conversion circuit 64. The speed estimator 68 uses, for example, model reference adaptive system (MRAS) principles of operation. The ω_(r) signal is combined with ω_(ref) to generate i_(qref), as explained above. The ω_(r) signal is also used by an output angle calculation circuit 70 to generate an output angle signal θ_(dq) that is used by the Clarke and inverse Clarke conversion circuits to translate between the stationary and rotating domains.

The output angle calculation circuit 70 includes a slip calculator 72 that determines slip frequency, ω_(s), as a function of i_(qref) and i_(dref). Specifically, the slip frequency may be defined using the following equation:

$\omega_{s} = {\frac{i_{qref}}{i_{dref}} \times k}$ where the value of k depends on the characteristics of the motor 12. The slip frequency is combined with the estimated rotor frequency, ω_(r), and that combination is integrated using an integrator 74 to generate the output angle signal θ_(dq).

As mentioned above, when operating in the current-based control mode the control circuitry 20 uses two independent feedback loops to control the strength and speed of the rotating magnetic field induced in the stator. A first feedback loop controls the strength of the rotating magnetic field and includes the Park 66 and Clarke 64 conversion circuits and the proportional-integral controllers 56, 58. A second feedback loop controls the speed of the rotating magnetic field and includes the Clarke conversion circuit 64, the speed estimator 68, the first proportional-integral controller 44, the field weakening controller 46 and the output angle calculation circuit 70. Thus, there is some overlap between the two feedback loops.

When the switch 28 is in the second position (indicated by solid lines in FIG. 2) the control circuitry 20 operates in the voltage-based control mode and controls the motor 12 by manipulating the voltage and frequency of the driving waveform rather than using the IFOC mode of operation employed in the current-based control mode. In the illustrated embodiment, V_(dref) is set to zero and V_(qref) is set to V_(rated), which may correspond to the maximum operating voltage. The maximum operating voltage will typically be the voltage available on the DC bus 32, but may also be related to the maximum voltage capacity of the inverter 42 and/or the motor 12.

In the voltage-based control mode, the control circuitry 20 operates the inverter 42 according to a target voltage (V_(rated)) rather than a target current. This mode of operation may be desirable, for example, where the motor 12 is operating at very high speeds and stator voltage is at or near the maximum voltage. In such situations, the voltage-based control mode allows greater power to be delivered to the motor 12 because the voltage margins associated the current-based control mode are eliminated.

Control circuitry 100 constructed according to another embodiment of the invention is illustrated in FIG. 3. The control circuitry 100 is identical to the control circuitry 20 illustrated in FIG. 2 and described above, except that the control circuitry 100 includes brake mechanisms for regulating the braking power of the motor 12 when the control circuitry 100 is operating in either the current-based control mode or in the voltage-based control mode. Therefore, for convenience similar components are assigned the same reference numerals and only the differences between the two embodiments will be discussed. Particularly, a current-based braking mechanism 102 is used when the circuitry 100 is operating in the current-based control mode and a voltage-based braking mechanism 104 is used when the circuitry 100 is operating in the voltage-based control mode.

The current-based brake mechanism 102 determines a difference between a change in the reference speed (Δω_(ref)) and a change in the estimated rotor speed (Δω_(r)). That difference is integrated using an integrator controller 106 to generate a differential speed signal Δi_(dref) that is combined with the i_(dref) signal when braking in the current-based control mode. Thus, the current-based braking mechanism 102 augments the i_(dref) signal according to a difference between the reference motor speed ω_(ref) and the estimated rotor speed ω_(r) thereby increasing motor losses and causing the motor to slow at a quicker rate. As the difference between the reference motor speed and the estimated motor speed increases, so does the value of Δi_(dref) and i_(dref).

A first braking switch 108 is operable to selectively connect the output of the current-based braking mechanism 102 to the i_(dref) signal combination point. The controller 22 may be configured to actuate the switch 108 to connect the current-based braking mechanism 102 to the i_(dref) signal combination point when the motor is in braking mode (i.e., when ω_(ref) is less than ω_(r)).

The voltage-based braking mechanism 104 augments the V_(rated) input to increase motor losses during braking when the control circuitry 20 is operating in the voltage-based control mode. The voltage-based braking mechanism 104 is similar in form and function to the current-based braking mechanism 102, described above, such that the braking signal increases with a difference between the reference speed and the estimated rotor speed. Augmenting the V_(rated) input causes the voltage applied to the motor to increase as well, thus increasing motor losses during braking and causing the motor to slow at a quicker rate. Increasing the voltage applied to the motor also increases the level of energy generated in the inverter and on the DC bus by the motor, which in turn increases the amount of voltage available to be applied to the motor, further increasing motor losses and braking power.

A second braking switch 110 is operable to selectively connect the output of the voltage-based braking mechanism 104 to the V_(rated) signal combination point. The controller 22 may be configured to actuate the switch 110 to connect the voltage-based braking mechanism 104 to the V_(rated) signal combination point when the motor is in braking mode (i.e., when ω_(ref) is less than ω_(r)).

As explained above, the amount of energy generated in the inverter 42 and the DC bus 32 by the motor 12 during braking will generally be limited by the rated capacity of the motor 12, the inverter 42 and/or the capacitor 36. When braking in either the voltage-based control mode or the current-based control mode, voltage generated on the DC bus 32 by the motor 12 is limited by the voltage limiting circuit 48. In the current-based control mode, the voltage limiting circuit 48 regulates both i_(qref) and the motor slip frequency to limit energy generation in the inverter 42 and on the DC bus 32 and thus avoid damage to those components. In the voltage-based control mode, the voltage limiting circuit 48 regulates only the slip frequency.

It will be appreciated by those skilled in the art that the braking mechanisms 102, 104 provide enhanced braking without compromising control of the braking process. The speed of the motor 12, for example, is tracked by the speed estimator 68 such that braking is enabled to decrease motor speed to any speed, including speeds other than zero. It may be desirable, for example, to slow the motor speed from 5,000 rpm to 3,000 rpm.

In operation, the controller 22 actuates the circuit control switch 28 to shift operation of the control circuit between the voltage-based control mode and the current-based control mode. The controller 22 further actuates the first brake switch 108 and the second brake switch 110 to use the brake mechanisms 102, 104 when in braking mode. Operation of the motor system 10 typically begins when the actual speed of the motor is zero and some non-zero reference speed is applied to the motor control system 16. The controller 22 actuates the circuit control switch 28 such that the switch 28 is in the first position to place the control circuitry in the current-based control mode. The control circuitry 20 generates a driving waveform causing the motor speed to gradually increase toward the motor reference speed. As the motor speed increases through the normal operating region, the stator voltage increases linearly with the speed of the motor 12, as illustrated in FIG. 4. If the reference motor speed ω_(ref) is in the normal operating region, the actual motor speed reaches the reference speed and stabilizes, and the control circuit 20 continues to drive the motor 12 at the reference speed in the current-based control mode.

If the reference motor speed ω_(ref) is in the medium speed range, the control circuitry 20 causes the motor speed to increase such that the stator voltage increases to and remains near the maximum available voltage as the actual motor speed reaches the nominal operating speed threshold. The motor speed continues to increase in the field weakening region as the torque-producing component of the stator current increases, the flux-producing component of the stator current decreases, and the motor torque decreases. The actual motor speed eventually reaches the reference speed and stabilizes, and the control circuit 20 continues to drive the motor 12 at the reference speed in the current-based control mode.

If the reference speed is in the high speed range of FIG. 1, the controller 22 continues to increase the motor speed in the current-based control mode until the estimated motor speed ω_(r) reaches the high speed range, wherein the controller 22 drives the switch 28 from the first position to the second position, shifting the control circuitry 20 from the current-based control mode to the voltage-based control mode. In the voltage-based control mode, the control circuitry 20 operates the motor 12 at the maximum rated voltage, as explained above. As long as the reference motor speed remains in the high speed range, the control circuitry 20 continues to operate in the voltage-based control mode.

While the motor 12 is operating in the high speed range, if the reference motor speed decreases the control circuitry 20 adjusts i_(qref) and i_(dref) causing the motor to operate at a negative slip frequency until ω_(r) decreases to correspond to ω_(ref). Operating at a negative slip frequency uses the driving waveform against the motor's movement and therefore acts as a brake. In this braking mode, the motor's movement generates electrical energy that may drive up the voltage in the inverter 42 and/or the DC bus 32. If the voltage in the inverter 42 and/or the DC bus 32 approaches the maximum voltage V_(dcmax), the voltage limiting circuit 48 responds by limiting the negative slip frequency to thereby limit the V_(dc) value and prevent damaging overvoltage.

If the control circuitry 20 includes the voltage-based braking mechanism 104, as soon as the reference motor speed decreases to a level below the estimated speed ω_(r), the controller 22 drives the second brake switch 110 to combine the voltage value V_(rated) with V_(qref) and thereby increase braking power. When the actual motor speed approaches or reaches the reference motor speed, the controller 22 drives the second brake switch 110 to disengage the braking mechanism 104 from the control circuitry 20.

As long as the reference speed is within the high speed range, the control circuitry 20 continues to operate in the voltage-based control mode. If the reference speed decreases from the high speed range to the middle or low speed range, however, the controller 22 switches operation of the control circuitry 20 from the voltage-based control mode to the current-based control mode by driving the switch 28 from the second position to the first position. If the motor system 10 brakes while in the current-based control mode, the controller 22 drives the first brake switch 108 to engage the current-based braking mechanism 102 as explained above, thereby augmenting the i_(dref) signal and increasing braking power.

The advantages of the present invention will be apparent to those skilled in the art. The design of the motor system 10 minimizes costs in that it does not require the use of a braking resistor, a regenerative front-end diode bridge, or motor shaft speed sensors. The system 10 operates at maximum efficiency by operating according to a target current at lower speeds where the current margin governs operation, and operating according to a target voltage at higher speeds where the voltage margin governs operation. Furthermore, the motor system 10 includes an enhanced braking system that can quickly brake the motor to any speed without losing the ability to measure the motor's speed.

Although the invention has been described with reference to the preferred embodiment illustrated in the attached drawing figures, it is noted that equivalents may be employed and substitutions made herein without departing from the scope of the invention as recited in the claims. The motor control system may be powered by a direct current source, for example, rather than the alternating current source as illustrated, wherein the rectifier may be omitted. Furthermore, the stator current and voltage values used in the control loops may be derived directly from motor input conductors rather than from the DC bus. 

Having thus described the preferred embodiment of the invention, what is claimed as new and desired to be protected by Letters Patent includes the following:
 1. A motor control system comprising: a power inverter; control circuitry configured to control the power inverter either according to a target voltage in a voltage-based control mode or according to a target current in a current-based control mode, the control circuitry including conversion circuitry for receiving first and second reference voltage signals with values corresponding to a rotating reference frame, converting the first and second reference voltage signals to third and fourth reference voltage signals with values corresponding to a stationary reference frame, and coupling the third and fourth reference voltage signals to the power inverter, and a switch that, in the voltage-based control mode, electrically couples the first and second reference voltage signals to zero volts and a voltage equal to a maximum capacity of the power inverter, respectively, and in the current-based control mode, electrically couples the first and second reference voltage signals to a flux producing reference voltage signal and a torque producing reference voltage signal, respectively; and a controller for operating the switch to switch between the voltage-based control mode and the current-based control mode.
 2. The motor control system of claim 1, further comprising a non-regenerative rectifier for receiving an alternating-current power waveform and generating direct-current power that is received by the power inverter.
 3. The motor control system of claim 2, further comprising a speed estimator for estimating a motor speed based on at least one characteristic of the direct-current power generated by the rectifier.
 4. The motor control system of claim 3, the speed estimator being the only source of motor speed information used by the motor control system.
 5. The motor control system of claim 3, the control circuitry including a first regulator for generating a reference current signal according to a magnitude of a target motor speed and/or a difference between the target motor speed and an estimated motor speed, the estimated motor speed being generated by the speed estimator; and a second regulator for adjusting the reference current signal according to a difference between a measured current and a value of the reference current signal.
 6. The motor control system of claim 5, the first regulator and the second regulator each including a proportional-integral controller.
 7. The motor control system of claim 1, the control circuitry including circuit components for performing indirect field oriented control.
 8. The motor control system of claim 1, the controller configured to switch operation of the motor control system from the current-based control mode to the voltage-based control mode when the motor control system is operating near a maximum voltage level.
 9. The motor control system of claim 8, the controller configured to switch operation of the motor control system from the current-based control mode to the voltage-based control mode in a field weakening range of motor operation.
 10. The motor control system of claim 1, further comprising a frequency generation circuit for determining a frequency associated with a motor drive signal.
 11. The motor control system of claim 1, wherein the control circuitry further includes a first regulator for receiving a differential signal i_(q) and generating the torque producing reference voltage signal, the differential signal i_(q) representing a difference between a reference torque producing current signal i_(qref) and a measured actual torque producing component i_(q) of a motor driving waveform, and a second regulator for receiving a differential signal i_(d) and generating the flux producing reference voltage signal, the differential signal i_(d) representing a difference between a reference flux producing current signal i_(dref) and a measured actual flux producing component i_(d) of the motor driving waveform.
 12. The motor control system of claim 11, wherein the control circuitry further includes a first current limiter configured to limit the values of the currents of the reference torque producing current signal i_(qref) and the reference flux producing current signal i_(dref) to a maximum current rating for a stator of a motor to be controlled by the motor control system.
 13. The motor control system of claim 12, wherein the control circuitry further includes a second current limiter configured to limit the value of the current of the reference torque producing current signal i_(qref) to a current rating corresponding to a maximum voltage of the power inverter before the value of the current of the reference torque producing current signal i_(qref) is limited by the first current limiter.
 14. The motor control system of claim 13, wherein the control circuitry further includes a voltage limiter circuit a first signal representing a maximum DC voltage of the power inverter and a second signal representing a DC voltage of a voltage supply for the motor, the voltage limiter circuit producing the current rating corresponding to a maximum voltage of the power inverter.
 15. The motor control system of claim 11, wherein the control circuitry further includes a third regulator for receiving a differential speed signal and generating the reference torque producing current signal i_(qref) based on the differential speed signal, the differential speed signal representing a difference between a reference motor speed and an estimated motor speed, and a fourth regulator for receiving a reference speed signal representing a reference motor speed and generating the reference flux producing current signal i_(dref).
 16. A method of operating a motor control system, the method comprising: generating a first motor drive signal according to a target current if a reference motor speed is within a first speed range, the first motor drive signal including a flux producing reference current signal and a torque producing reference current signal; generating a second motor drive signal according to a target voltage if the reference motor speed is within a second speed range, the second motor drive signal including a zero volt signal and a voltage signal equal to a maximum capacity of a power inverter; and automatically switching operation of the motor control system between generating the first motor drive signal and generating the second motor drive signal as the target speed changes between the first speed range and the second speed range.
 17. The method of claim 16, further comprising using a non-regenerative rectifier to receive an alternating-current power waveform and to generate direct-current power, communicating the direct-current power to a power inverter, and controlling the power inverter with either the first motor drive signal or the second motor drive signal.
 18. The method of claim 17, further comprising estimating a motor speed using at least one electrical characteristic of the direct-current power.
 19. The method of claim 18, the step of generating the first motor drive signal including adjusting a value of a reference current signal according to a target motor speed and/or a difference between the target motor speed and the estimated motor speed, and adjusting the value of the reference current signal according to a difference between an estimated current and a target current.
 20. The method of claim 19, the step of generating the first motor drive signal including increasing the value of the reference current signal if the target motor speed is greater than the estimated motor speed and decreasing the value of the reference current signal if the target motor speed is less than the estimate motor speed, and increasing the value of the reference current signal if the target current is greater than the estimated current and decreasing the value of the reference current signal if the target current is less than the estimated current.
 21. The method of claim 16, further comprising receiving first reference voltage values according to a rotation reference frame from either of the first or second control circuits, converting the first reference voltage values to second reference voltage values according to a rotating reference frame, and communicating the second reference voltage values to the power inverter.
 22. The method of claim 16, the step of generating the first motor drive signal including performing indirect field oriented control.
 23. The method of claim 16, further comprising switching operation of the motor control system between generating the first motor drive signal and generating the second motor drive signal when the motor control system is operating near a maximum voltage level.
 24. The method of claim 16, further comprising switching operation of the motor control system between generating the first motor drive signal and generating the second motor drive signal when the motor is operating in a field weakening range.
 25. A motor control system comprising: a power inverter; control circuitry configured to control the power inverter either according to a target voltage in a voltage-based control mode or according to a target current in a current-based control mode, the control circuitry including a first regulator for receiving a differential speed signal and generating a torque producing component i_(qref) of a reference current signal based on the differential speed signal, the differential speed signal representing a difference between a reference motor speed and an estimated motor speed, a second regulator for receiving a reference speed signal and generating a flux producing component i_(dref) of the reference current signal; a third regulator for receiving a differential i_(q) signal and generating a torque producing component V_(q) of a reference voltage signal, the differential i_(q) signal representing a difference between i_(qref) and a measured actual torque producing component i_(q) of a motor driving waveform; a fourth regulator for receiving a differential i_(d) signal and generating a flux producing component V_(d) of the reference voltage signal based on the differential i_(d) signal, the differential i_(d) signal representing a difference between i_(dref) and a measured actual flux producing component i_(d) of the motor driving waveform; and a controller for switching operation of the motor control system between the voltage-based control circuit and the current-based control circuit.
 26. The motor control system of claim 25, the control circuitry including an input for receiving a maximum voltage value associated with the motor when operating in the voltage-based control mode.
 27. The method of claim 16, the step of generating the first motor drive signal including limiting the current values of the reference torque producing current signal and the reference flux producing current signal to a maximum current rating for a stator of a motor to be controlled by the motor control system. 